Method for manufacturing prepackaged molding compound for component encapsulation

ABSTRACT

A method and apparatus for providing a prepackaged mold compound for use in encapsulating integrated circuit die and leadframe assemblies. A piece of mold compound 71 is placed in a receptacle 91 in a bottom mold chase 83. The receptacle 91 is coupled to a group of die cavities 85 by runners 87. Leadframe strip assemblies containing leadframes, integrated circuit dies, and bond wires coupling the leadframes and dies are placed over the bottom mold chase 83 such that the integrated circuit dies are each centered over a bottom mold die cavity 85. A top mold chase 93 is placed over the bottom mold chase and the prepackaged mold compound 71. The prepackaged mold compound 71 is a piece of mold compound 73 packaged in a plastic film which has sealed edges 77. The edges are peelable seals, which are released during the molding process. The mold compound 73 is then forced through the seals during the molding process by the pressure applied by a plunger 101. The plunger 101 can be applied using variable speed and pressure to control the rate the mold compound 73 fills the cavities in the top and bottom mold chases, thereby avoiding voids in the completed packages and minimizing wire sweep of the bond wires of the integrated circuit assemblies.

FIELD OF THE INVENTION

This invention relates generally to the field of integrated circuits,and more particularly to the encapsulation packaging of integratedcircuits using transfer molding techniques.

BACKGROUND OF THE INVENTION

In producing integrated circuits, it is desirable to provide packagedintegrated circuits having plastic or resin packages which encapsulatethe die and a portion of the lead frame and its leads. These packageshave been produced a variety of ways, a few of which will be describedhere.

Conventional molding techniques take advantage of the physicalcharacteristics of the mold compounds. For integrated circuit packagemolding applications, these compounds are typically thermoset compounds.These compounds consist of an epoxy novolac resin or similar materialcombined with a filler, such as alumina, and other materials to make thecompound suitable for molding, such as accelerators, curing agents,fillers, and mold release agents.

Transfer molding operations have three stages which correspond to thethree phases of viscosity. First there is a preheat stage required tomove the mold compound from its hard initial state to the low viscositystate. Second is a transfer stage, where the compound is low inviscosity and easily transported and directed into cavities and runners.This transfer process must be rapid and be completed before the moldcompound begins to set. Finally there is a cure stage that occursfollowing the transfer stage.

There are several critical requirements that must be met in acommercially successful package molding operation. For example, thecavities must be completely and uniformly filled, the so-called"balanced fill" requirement. Using conventional single plunger molds ofthe prior art, the balanced fill is difficult to perform uniformlyacross the large mold using the single pot and the long primary runnersto transport the mold compound. A problem commonly observed in a singleplunger, single pot mold operation using such a mold is an unacceptablevoid rate. Voids are areas within the mold cavity that are not filledwith compound. These can be areas where the compound fails to flow orwhere air or other materials are trapped and cause hollow spaces in thepackaged part. Voids can be produced if the transfer rate of the moldcompound is too slow during the molding process or if air or moisture istrapped in one or more the cavities during the transfer stage.

A second critical requirement is that the wire sweep defect rate beminimized below an acceptable level. Wire sweep occurs as the moldcompound enters the cavity through the gates. The mold compound is denseand pulls at the fine wires that couple the bond pads of the die to theleads of the lead frame. These wires will bend under the pressure due tothe flow of mold compound. As an example, suppose that in a typical leadframe and die assembly for a high pin count device, an average wiresweep after packaging of less than 6% is specified. A straight line fromthe lead frame lead to the bond pad has a sweep of 0%. So if afterassembly and mold any wires on a packaged unit are found to have morethan 6% sweep, the unit is out of specification, and is considered to bea defective unit. Wire sweep is specified as a maximum allowableparameter and is of considerable concern in production of integratedcircuits, because if the bond wires are moved too much a wire shortbetween two or more adjacent bond wires often occurs. Alternatively,bond wires sometimes break away. Either condition results in a faultyunit.

Although the wire sweep defect rate which is observed in the singleplunger molding presses is adequate for producing low to moderate pincount DIP and flat quad packaged devices, as the device pin countscontinue to increase and lead frames become finer in lead to lead pitch,the wire sweep parameter becomes increasingly critical. While it ispossible to build 200 pin flat quad devices using these techniques, asthe pin count goes towards 400 pins and beyond, the pitch between leadswill become smaller, and the prior art transfer molding presses using asingle mold pot will no longer be economically suitable, due to the lowyield and high wire sweep defect rates.

A further disadvantage with a single plunger mold and pellet compoundarrangement is that the performance in the two critical areas areinversely dependent on each other. That is, in attempting to perfect themolding process using a single plunger mold, it has been observed thatsteps taken to reduce wire sweep defects typically increase the voidrate, and vice versa. In other words, if the wire sweep defect rate islowered the void rate tends to increase. The wire sweep rate can belowered, for example, by slowing the transfer rate of the mold compoundinto the cavities. This tends to increase the void rate. Voids can bereduced by increasing the flow rate into the cavities, but this willtend to increase the wire sweep defect rate.

It has been further observed that the wire sweep and void problems tendto be more severe as the number of cavities and the distance of runnersincreases. Nonuniform fill can occur along a lengthy runner having manycavities. The cavity closest to the pot will have a faster fill ratethan the others. The cavity farthest from the pot will tend to fill atthe end of the transfer period, and the rate will be lower because a lotof the compound has been diverted to other cavities and because thecompound is starting to harden. As a result, difficult and timeconsuming fine tuning of each mold press is required to establish anoperation mode which will fill all of the cavities at an acceptablerate, during the low viscosity period, without increasing wire sweepdefects to an unacceptable level, particularly for those cavitiesclosest to and farthest from the mold pot.

Further, the use of the thermoset molding compound results in a processwhere the sprue, flash or waste that remains in the pot, the runners andbetween the devices themselves cannot be reused. Thermoset materials canonly be used once in a molding operation, so the excess material must bediscarded. Thus the sprue and waste left in the long runners and in themold pot cannot be recycled, making waste particularly costly.

Also, the conventional molding compound acts as a strong abrasive.During molding, the mold compound is forced out of the mold pot and intothe primary runners. The abrasive nature of the mold compound results inrapid wear of the mold pot and the runners, and the plunger or ramitself. This results in expensive rework or replacement of the moldchases and plungers on a frequent basis.

An alternative prior art approach for reducing the problems known to thesingle plunger molding presses of the prior art is to construct amultipellet, multiplunger mold station to replace the single plungersystem.

In a multiplunger molding operation, each of the many mold pots receivesa so called "mini-pellet" of mold compound. Each mold pot serves only afew cavities, typically one or two cavities. The press is a more complexpress than that of the single plunger mold, and has a plunger for eachof the mold pots. The plungers may operate from the top or fromunderneath the mold. The individual plungers are used to start thetransfer process, the cavities fill with mold compound as the plunger ispushed into the mold pot, and the transfer phase is completed in a fewseconds.

The multiplunger mold process has some advantages over the single potmolding process. The use of the smaller pellets and the shorter runseliminate the long runners and nonuniform fill times associated with asingle plunger press. The pellets used are smaller and therefore do notrequire preheating, as the mold platens can provide sufficient heat totransition the mini-pellets into the low viscosity state. The wire sweepdefect rate can be lowered by providing exact control of the plunger orram insertion rate, so that the fill is done at a speed which preventsvoids while minimizing wire sweep problems. An automated multiplungerpress controller can be added that can individually vary the operationof each plunger, if necessary, to obtain optimal results.

The nonuniform fill and wire sweep problems associated with the cavitiesnearest and farthest from the single center pot of the single plungermold presses are reduced or eliminated. Mold compound waste is reducedby the shorter runners.

The disadvantages of the multiplunger molding process are primarily thatit requires the use of the mini-pellets. The mini-pellet form of themolding compound is far more expensive than per kilogram than the singlelarge pellets used by the single transfer mold. Also, the multiplungermolding station is extremely expensive to manufacture, operate andmaintain. The automation of a press with so many plungers is morecomplex and expensive than the single mold press.

In addition to the added costs, the multiplunger molding station has alower parts per hour throughput than for a conventional single pot moldpress. The multiple plunger molding system requires complex control andloading and unloading mechanisms. The result is that each station haslower overall throughput than a single plunger mold station, althoughtighter process control can be achieved. Because the throughput islowered, additional stations are needed to maintain the same relativelevel of productivity. High productivity is required to keep the unitcosts low. The need for additional expensive and complex moldingstations increases the cost disadvantages for the multiplunger moldingsystems.

Both single plunger and multiplunger mold presses have otherdisadvantages that are common. The mold compound is an abrasivematerial. The mold pot and the primary runners receive an abrasive forceeach time the press is operated. These areas wear quickly and theexpensive mold chases must be replaced periodically as a result.

Also, both processes require pelletized mold compound. This material isfairly difficult to produce in the large form, and even more expensiveto produce in the minipellet form. The compound is extruded into a rod,which is powdered, and the powder is then pelletized. This is anexpensive and complex manufacturing process.

Both pellets and mini-pellets are subject to contamination by moistureand air. It is necessary to perform the molding process under pressureto eliminate trapped air and prevent the formation of voids. Moisturecan become trapped in either form of pellet. Moisture contamination ofthe molding compound can result in additional voids and scrappeddevices. Moisture contamination also contributes to package crackingduring cure and afterwards to early failure of devices.

U.S. Pat. No. 5,098,626, issued Mar. 24, 1992, and entitled "Method forPacking a Measured Quantity of Thermosetting Resin And Operating a Moldfor Encapsulating a Component", and herein incorporated by reference,provides another alternative wherein the mold compound is packaged inindividually sealed units. These units each contain liquid mold compoundin a quantity needed for a single cavity or pair of cavities forintegrated circuit packages. Each package is a bag or tube containingliquid mold compound and ending in a bulge or sprout. During molding thebulge or sprout is placed at the end of a runner which feeds a cavity.As the molding process begins, the sprout is cut and the mold compoundis pressed out of the bag into the cavity by individual, multipleplungers.

The '626 patent approach is similar to a conventional multiplunger moldsystem in that small quantities of mold compound, each of which areindividually loaded, are provided. The patent provides a moisture andcontamination free packaging system which can be used with an automatedloading system. However, like the mini-pellets, many of these bags arerequired for each run. The abrasion problems are reduced, because thepots and plungers are protected by the packaging. Also, improved uniformfill and reduced wire sweep are possible. But the throughput problemsand increased expense for each molding station remain, and the costs foreach press are increased further by the added complexity. Also, thepackaging of the mold compound in small quantities each in an individualpackage may lead to an expensive raw material for molding.

Accordingly, a need thus exists for a mold compound and molding systemwhich eliminates the problems of the prior art transfer molding systemswhile retaining a high throughput rate, low raw material costs, andwhich is simple to operate, maintain, and uses molding stations that arerelatively inexpensive to build. The new system should be compatiblewith existing single pot transfer mold presses to allow a retrofittingof existing integrated circuit assembly lines. The system should reducewaste of mold compound and reduce the abrasive impact of the moldcompound on the equipment used. The new molding system should provideuniform cavity fill and reduced wire sweep defect rates.

The new molding compound should be free of impurities, air and moistureto reduce void and package cracking problems. It should be in a formthat is compatible with automatic loading and unloading systems. Itshould be premeasured and have excellent storage durability. It shouldbe economically competitive with the pellets and mini-pellets of theprior art. The invention described herein addresses these needs.

SUMMARY OF THE INVENTION

A system for transfer molding the packages of integrated circuits usingpre-packaged mold compound packaged in a protective plastic is provided.The plastic packaging is sealed at the edges by a heat seal or similarmethod that results in a peelable seal that opens during molding. As thepackage is heated in the mold, the edge seals become flexible. The moldcompound can then be pushed through the edge seals at places adjacent tothe mold runners during the transfer molding process. The protectivepackaging ensures that the mold compound is free from moisture and aircontamination and is easily produced, stored and shipped. The mold andmold compound packaging are designed so that the packaged mold compoundfits easily into the mold and is easily removed. The mold compound canbe a variety of shapes and sizes as the particular mold design dictates.

In a first preferred embodiment, an improved mold design is used incombination with prepackaged mold compound inserts. The mold chasesinclude rectangular receptacles for receiving the mold compound inserts.A rectangular plunger is provided for each of the receptacles. Eachpackage cavity is equidistant from the receptacle containing moldcompound, providing improved uniformity of fill and allowing forcomplete fill of the cavities with reduced wire sweep as compared to thetransfer molds of the prior art. The rectangular plunger is insertedinto the mold compound receptacle and the mold compound is forcedthrough the edge seals of the protective packaging into short runnerscoupling the mold receptacle to the cavities. The number of devicespackaged per run is increased because the mold pots of the single ormultiple plunger molds of the prior art are eliminated, providingadditional area for die cavities.

The mold compound insert is placed inside the mold receptacle within theprotective package, so that the equipment abrasion problems associatedwith conventional prior art transfer molding operations are reduced oreliminated. Since the runners are shortened, the amount of mold compoundwhich is flash or sprue for each run is reduced, thus reducing waste andlowering production costs. The improved mold design is compatible withexisting automated pick and place loading and unloading systems forincreased automation and improved throughput. The molding stationrequires only a few plungers and is inexpensive to build and tomaintain. Existing molding equipment may be retrofitted to use the newsystem. The mold system is easily combined with a process controller toachieve tight process control, and the use of the prepackaged moldsystem with a process controller results in a mold process with balancedcavity fill, reduced wire sweep and low void defect rates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prepackaged mold compound package of the invention;

FIG. 2 depicts a bottom mold chase of the mold system of the invention;

FIG. 3 depicts top mold chase of the mold system of the invention;

FIG. 4 depicts the plunger used with the top and bottom mold and theprepackaged mold compound of the invention;

FIG. 5a depicts a completed mold compound package of the invention;

FIG. 5b depicts a cross section of the completed mold compound package;

FIG. 5c depicts the details of an end seal of the completed moldcompound package of the invention;

FIG. 6 depicts one of the steps of the process of making the moldcompound of the invention;

FIG. 7 depicts one of the steps of the process of making the moldcompound of the invention;

FIG. 8 depicts one of the steps of the process of making the moldcompound of the invention;

FIGS. 9A and 9B depicts one of the steps of the process of making themold compound of the invention;

FIG. 10 depicts one of the steps of the process of making the moldcompound of the invention;

FIG. 11 depicts one of the steps of the process of making the moldcompound of the invention; and

FIG. 12 depicts a cross sectional view of a transfer molding processusing the prepackaged mold compound of the invention.

Corresponding numerals are used for corresponding elements in thedrawings, unless otherwise indicated in the text.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a prepackaged mold compound insert 71 in a firstpreferred embodiment of the mold compound of the invention. Many othershapes are possible and will still obtain all of the advantages of themolding system of the invention. The mold compound 73 may be composed ofconventional resin or resin filler mold compound in a solid orsemi-solid form. Alternative molding compound compositions may be used.The mold compound 73 may be made from powdered conventional moldcompound, or preferably directly from extruded mold compound, therebyeliminating the need for the expensive pelletizing steps required forthe pellets of the prior art mold compounds.

The mold compound 73 is packaged in a prepackaged plastic package.Sleeve 75 surrounds three sides of the compound and the ends. Ends 76 ofsleeve 75 are sealed. The compound 73 is covered by top 74, which is apiece of plastic film which is wider than the mold compound 73 andprovides a lip on either side of the mold compound 73. Sleeve 75 is asecond piece of plastic film that wraps around the sides and bottom ofthe mold compound 73 and is sealed at the ends 76. Edges 77 are sealsthat couple the sleeve 75 to the top 74. Sleeve 75 and top 74 may be ofthe same material or of differing materials.

The plastic package of FIG. 1 may be composed of many materials, but thematerial must meet certain requirements. The package should not crack orbecome permeable during storage or before molding within the specifiedtemperature ranges for storage. The seal at the edges 77 should notdegrade during storage, at least the seal at the edges should not becomepenetrable during storage so that the mold compound leaks or iscontaminated by substances coming into the package. The seal at the endsshould also remain impenetrable during storage. The package should beable to maintain a vacuum during storage. The package should notcontaminate the equipment or the environment during storage or duringthe molding process.

The plastic package 71 of FIG. 1 provides the advantages of making theprepackaged mold compound inserts impervious to contaminants such aswater that could interfere with the molding process and the reliabilityof the resulting packages. Since the inserts are self-packaged inplastic, storage and shipping packing materials may be inexpensive andno additional protective layers may be needed. The protection of themold compound from moisture prevents many of the package crackingproblems and voids associated with moisture contaminated mold compound.The top 74 may be opaque and may carry labeling information in text andmachine readable forms, such as bar codes or universal product code(so-called UPC) labels. This labeling on the mold compound package 71provides an easy mechanism for checking that the correct type of moldcompound is being used for a particular packaging operation. Also, theplastic packaging affords the opportunity to use alternative moldcompounds instead of the resin or resin filled compounds known to theprior art, because the mold plunger and mold receptacle or pot are notin direct contact with the compound.

FIG. 2 depicts a portion of a bottom chase for transfer moldingintegrated circuit packages, such as, for example, DIP or flat quadpackage type high pin count integrated circuit packages, using the moldcompound package of FIG. 1. Bottom mold chase 81 holds two cavity bars83, each of which has several die cavities 85 coupled to primary runners87, and each cavity having a gate 89. A rectangular mold compoundreceptacle 91 is provided that extends through the mold cavity bars 83and the mold chase 81. This receptacle 91 is open at the bottom forallowing a plunger or ram to enter the mold chase and to apply pressureto a mold compound package of FIG. 1 resting at the top of receptacle91, to force the mold compound into the runners and the cavities. Atypical mold system would include two to four of these mold chase pairs81, so it would have two to four receptacles 91, and a correspondingarray of cavities along both sides of each receptacle. For certain typesof packages, as many as six to eight chases may be required for a singlemold.

FIG. 3 depicts a portion of a top mold chase 92 for use with the bottommold chase 81 of FIG. 2 and the mold compound package of FIG. 1. In FIG.3, top mold chase 92 carries mold cavity bars 93, each of which includesa row of cavities 95 which are positioned to be placed over the bottommold chase cavities 87. Delivery runners 97 are positioned with an outerend which will meet an associated primary runner 87 in the bottom moldchase, and an inner end which will lie over the mold compound receptacle91.

FIG. 4 depicts the plunger 101 which is used with the top mold chase 92of FIG. 3 and the bottom mold chase 82 of FIG. 2. The top of plunger 101is sized so as to fit within the receptacle 91 in the bottom mold 82.The top of the plunger is operable to compress the mold compound packageagainst the top mold chase 92 in an even manner along the insert of moldcompound in the manner described below. The top of plunger 101 isbeveled and machined so that a small area at the edge of the top andsides are spaced beneath the top surface a short distance to form tip103. The tip 103 compressed against the sides of the plastic package andas the plastic is compressed, the plastic can deform into this spacingand the package will continue to compress without holding the topsurface of plunger 101 away from the top mold surface.

In operation, the pre-packaged mold compound molding system includingthe packaged mold compound shown in FIG. 1, the bottom mold chase ofFIG. 2, the top mold chase of FIG. 3, and the plunger of FIG. 4,operates as follows. The mold is opened so that the top mold and topmold chase is separated from the bottom mold and bottom chase and thebottom mold cavity bars 83 may be accessed from above. Lead frame stripshaving lead frame and die assemblies are placed over the bottom moldcavity bars 83 such that a single leadframe and die with its bond wiresis centered over each cavity 85. A packaged mold compound insert 71 isplaced in each receptacle 91 in the bottom mold. These placements arepreferably performed by an automatic pick and place mechanism as isknown in the prior art, but alternatively may be performed manually.

The bottom and top mold chases 83 and 93 are heated as in theconventional transfer molding stations, and the heat in the mold itselfis sufficient to transition the mold compound into the transfer phasewithout preheating, so the preheating step required with the prior artsingle pot molding press is eliminated.

After the bottom mold chases 83 are loaded and the molding inserts arein place in the bottom mold receptacles, the mold is closed and the topmold chases are brought into contact with the leadframe and dieassemblies and the mold compound packages. Runners 97 in the top moldchases 93 are now positioned so that the inside ends of these runnersare positioned over the edges of the mold compound within thereceptacles 91.

The mold compound packages may be heated for a short time to reach thelow viscosity state. While this heating is taking place, the heat sealin edges 77 of the molding compound pencil packages opens, that is theheat relaxes the seal so that the seal is now penetrable by the moldcompound. The mold is typically heated to a temperature of 175 degreesCelsius when conventional resin or resin filler molding compound is usedin either powdered solid or liquid states.

After the heat seals are relaxed and the mold compound enters the lowviscosity state, the plunger 101 of FIG. 10 is applied. In a preferredembodiment, the plunger 101 travels through the bottom mold chase andinto the bottom mold receptacles 91, compressing the mold compoundinserts 71 from underneath. Alternatively, the inserts could becompressed from above, with the receptacles formed in the top moldplaten. In this case the mold compound inserts would be loaded with thetop plastic layer 74 down, that is adjacent the bottom mold chases.Either arrangement will work to transfer the mold compound into theprimary runners.

In this embodiment, the mold compound is compressed by the action of theplunger 101 and as it is compressed the mold compound will begin to pushat the edges of the receptacle 91, and the mold compound will push atthe edges of the mold compound package. As the only exits available tothe mold compound are the delivery runners 97 in the top mold cavitybars 93, the compound will pass through the now penetrable heat seal atthe edge of the plastic package 71 and into the runners 97. The deliveryrunners each feed a primary runner 87 in the bottom mold chases 83. Acircular coupling area at the inner end of the primary runners meets theouter end of the delivery runners 97, and the mold compound istransferred to the primary runners 87. The mold compound then enters thecavities 85 over the gates 89, and begins filling the individual packagecavities 85.

After the cavities are filled with the compound, the molding processcontinues as a conventional transfer molding process. After the packagesare cured, if required for the particular molding compound, the top moldis moved away from the bottom mold. Small plungers, not shown in thefigures, are activated to release the packaged devices from the cavities85, and the sprue or flash is released from the runners 87. The moldcompound package is now almost empty and resting in the receptacle 91,and it too is removed. The need to clean the receptacle 91 and theplunger 101 is greatly reduced over prior art molding systems becausethe packaging of the mold compound insert 71 serves to isolate theplunger 101 and the receptacle 91 from the mold compound.

A critical element to the operation of the molding process using theprepackaged molding compound is the packaging material. The requirementsfor the packaging of the mold compound have been established for anintegrated circuit assembly process using industrial standardrequirements for molding compounds and for the resulting integratedcircuit packages. The plastic package should not create appreciableresidue or glue like substances in the mold during molding. The moldcompound packaging should not contaminate the mold runners orreceptacle. The material used in the packaging should not add to ioniccontamination of the resulting packages, that is the material should nothave an ionic content higher than that of the molding compounds in usein the integrated circuit packaging art. The packaging material shouldmelt during the molding process, so it must have a melting temperatureat least ten degrees Celsius over the molding temperatures. Typically,the material needs to have a melting point of greater than 200 degreesCelsius. The material should only allow the molding compound to exit thepackage at selected points adjacent to the runners, and it should notopen prematurely during the preheat phase of the molding operation. Sothe edge seals should not open and emit molding compound prematurely.However, once the edge seals are penetrable the mold compound should beable to flow out of the package with a minimum of resistance to flow.The material should not tear in normal handling or shipping, but shouldhave the capacity to stretch into the runners when compressed during themolding process as described above. The material should be capable ofvacuum sealing and of maintaining the vacuum during storage.

Plastic films, such as those used in food storage, freezing andpreparation, are particularly well suited to this application. Themelting point, strength, vacuum capability and moisture and air barrierrequirements for the mold compound packaging are all met by such films.The films are inexpensive and easy to purchase and use in a productionenvironment. One preferred film is MYLAR™ polyester film, such as forexample MYLAR™ 40 XM 963-AT, a polyester film for packaging availablefrom E. I. Du Pont, Du Pont de Nemours Int. S.A., Geneva, Switzerland;or Du Pont (U.K.) Ltd, Maylands Avenue, GB-Hemel Hempstead, England.Another preferred film is sold under the trade name ICI™. Similar filmsare commercially available from a variety of vendors. The top and sleevefilms can be of differing material so long as a good edge seal, which isimpenetrable during storage and handling and becomes penetrable eitherunder heat or pressure or both, can be made between the two materials.

Once the appropriate material is selected, the film must be applied tothe mold compound to create the necessary packaged mold compound insert.FIGS. 5a, 5b and 5c depict in some detail the completed, packaged moldcompound insert.

FIG. 5a depicts the completed mold compound insert. Package 71 is aplastic polyester film package of two pieces, a lid and a top. Ends 75are permanently sealed. Pencil shaped mold compound 73 is typically,although not restricted to, a resin filled or resin mold compound asknown in the art. Alternatives include epoxies, plastics, and otherencapsulants.

FIG. 5b depicts the cross section taken along line A--A of FIG. 5a. Edge77 forms a seal between the sleeve 75 and the top 74. This seal becomespenetrable during the transfer phase of the molding process, and soshould release under heat, pressure, or both heat and pressure. The sealacts as a barrier to moisture and air and other contaminants duringshipping, storing and normal handling of the mold compound. A heat sealformed between sleeve 75 and top 74 has been found to be an effectiveembodiment. Alternatives include adhesives such as glues and tapes, andpressure seals. The seal should not restrict flow of the compressed moldcompound once the seal is opened for the transfer process. The sealshould not contaminate the mold, that is when the package is removedafter the molding process the seal should be with or inside the packageand not attach itself to the plunger or mold surfaces.

FIG. 5c depicts a detailed view of one end of the package 71. Moldcompound 73 terminates before the end of sleeve 75. A crimped end 53brings the ends of sleeve 75 together. End 53 is sealed with sealant 51.In a preferred embodiment, sealing tape such as a double sided tape isused. Again these tapes are but one alternative, glues, pressure, heat,and other sealing techniques may be used. The end seal should remain inplace up to a temperature of about 180 degrees Celsius. The end sealshould be applicable quickly and in a vacuum environment. The end sealshould not contaminate the mold or other equipment, and should adhere tothe film material. A preferred embodiment uses precut pieces of doublesided sealing tape. Alternatives are epoxies, adhesives, sealingcompounds, heat seals, crimped seals and other well known seals forplastics.

FIGS. 6-11 show the steps performed to manufacture the packaged moldingcompound insert shown in FIGS. 1 and 5a-c. In FIG. 6, the application ofthe end seals to a polyester film sleeve 75 cut to an appropriate lengthis shown from a top view perspective. The end seals 76 are placed on theupper surface of sleeve 75 in FIG. 6, which will become the interiorsurface of sleeve 75 ion the completed package. Labels A and B areprovided as orientation references for subsequent illustration anddiscussion in the following drawings.

FIG. 7 shows the sleeve 75 placed into a loading block 61 for receivingthe mold compound. FIG. 7 is an end view, and labels A and B from FIG. 6are repeated in FIG. 7 to clarify the orientation of the film sleeve 75in FIG. 7. Cavity 63 is of a width and depth determined by the size ofthe finished package as needed for the particular molding operation.Sleeve 75 is placed so that the sides are extended out to form a lip oneither side of the top of cavity 63 in loading block 61.

FIG. 8 repeats the end view of FIG. 7 and shows the sleeve 75 nowreceiving the piece of mold compound 73. The cavity 63 is shaped toreceive the mold compound 73 and so the sleeve 75 now pushes into thecorners of cavity 63.

FIGS. 9a and 9b show the formation of the heat seals between the topfilm piece and the sleeve of the plastic film package. FIG. 9a isanother end view as are FIGS. 6 and 7. FIG. 9b is a top view. To formthe heat sealed edges of the package the following steps are followed.Top piece 74 is placed over the sleeve 75, compound 73, and loadingblock 61. Sealing blocks 65 are then placed over the top 74 a shortdistance away from the top edges of cavity 63. Heat is applied tosealing blocks 65 for the time required to cause sleeve 75 and top 74 tobecome sealed together in the heated region. The heat seals areunaffected by moisture and other contaminants but should becomepenetrable when the seals are again heated.

FIG. 10 depicts the package film cutting step. After the heat seals arecomplete, lengthwise cuts in the edges of sleeve 75 are made outside thearea of the heat seals. The width of the top 74 is determined by themold design to be used, as the lip formed by the top 74 is used to holdthe mold compound in the proper place in the mold receptacle during themolding process. Variations in the receptacle and mold design are easilyaccommodated by varying the mold compound package top 74 width at thiscutting step.

FIG. 11 depicts the result of the next step required to make thepackaged mold compound, the vacuum sealing step. The mold compound 73with the sleeve 75 still open at the ends is placed into a vacuumchamber. The vacuum is reduced to a pressure of about 20 millibars. Theend seals are then mechanically sealed to form the package as shown inFIG. 11. Vacuum reduces or eliminates any air from the packaged moldcompound insert. This is advantageous because air trapped between thepackage and the mold compound can be pushed out into the runners duringthe molding process and can create voids in the completed package.Alternatives to the vacuum sealing step include a tight wrapping stepwhich would cause the plastic sleeve to tightly wrap the mold compoundand prevent air from being packaged with it. Shrink wrapping could beused, for example.

FIG. 12 depicts the transfer stage of molding using the prepackaged moldcompound package of FIG. 11. In FIG. 12, prepackaged mold compound 71 iscompressed by plunger 101 and tip 103. The mold compound is forced intodelivery runner 97 and primary runner 87, and over gate 89. Integratedcircuit die 106 lies on a die pad 105 inside the cavity 104 formed bytop cavity 95 and bottom cavity 85, with leadframe 107 also inside thecavity 104.

In operation, the function of the beveled tip 103 can now be seen, asthe plastic package sides are compressed into the slots machined intothe plunger 101 so that the compression can continue withoutinterference. The compound travels into the delivery runner 97, theninto the primary runner 87, over the gate 89, and into the cavity 108formed by the top and bottom chase cavities 95 and 85.

After the cavities 104 are filled with the compound, the molding processcontinues as a conventional transfer molding process. A curing time maybe required to complete the packages, alternatively some mold compoundsmay instantly cure. After the packages are cured, the top mold is movedaway from the bottom mold. Plungers are activated to release thepackaged devices from the cavities 85, and the sprue or flash isreleased from the runners 87. The mold compound package 71 is now emptyand resting in the receptacle 91, and it too is removed. The need toclean the receptacle 91 and the plunger 101 is greatly reduced becausethe packaging of the invention serves to isolate the plunger and thereceptacle from the mold compound.

The plungers 101 are easily controlled with a variable rate ofcompression to achieve a tight process control parameter during thetransfer phase. This process control leads to uniform fill of thecavities, which are evenly spaced and equidistant from the source of themold compound, and the transfer speed can be controlled to eliminatevoids while minimizing pad tilt and wire sweep defects. The transferspeed and transfer pressure can be controlled by fitting an independentprocess controller circuit to the mold system to allow multi-step,variable speed and variable pressure capability. This equipment can beretrofitted to an existing mold press.

Many advantages accrue as a result of using the prepackaged moldingcompound of FIG. 1 with a mold designed to efficiently use theprepackaged molding compound, such as the molding system of FIGS. 2-4.An advantage of the molding system of the invention is that is providesbalanced fill capability. It can be seen from FIGS. 2 and 3 that eachprimary runner and secondary runner is the same length. Because thecavities are all equidistant from the source of mold compound,receptacle 91, the problems of nonuniform fill and wire sweep associatedwith the single pot mold systems of the prior art are eliminated usingthe molding system of the invention.

Further, because the mold compound 73 is prepackaged in a plasticencapsulation, the mold receptacle, the plunger, and to some extent theprimary runners are protected from the abrasive mold compound, so thatthe wear rate is greatly reduced. This results in longer mold life andreduced repair and replacement costs over the life of the mold surfaces,thus lowering the unit cost.

It can further be seen that as another advantage of the use of theinvention, the mold receptacle 91 takes a small amount of area comparedto the large single pot and primary runners of the single pot transfermolds of the prior art. This is an advantage in that additional space isavailable for cavities and additional units may be molded during eachrun. The density for the system is improved over the prior art.

The mold design and mold compound is also compatible with existingautoloading systems for transfer molds, so that the mold system may beretrofitted into an existing automated transfer mold assembly line for areasonable cost. The plunger design and mold design results in a needfor two to four plungers per mold, which is cheaper to build andmaintain than the multiple plungers needed for a mini-pellet multipleplunger system.

Further advantages are that the mold compound insert packages arereasonable in cost and may be produced in volume for a lower price perkilogram than the mini-pellets required by the prior art or the multiplepackets required by the sprouted bag encapsulation system. It isbelieved that as the volume increases the mold compound inserts may beproduced at a price similar to the pellets of the single pot moldingsystems of the prior art.

Because the throughput rate of the mold system is high, the number ofstations required for a particular throughput rate is lower than themultiple plunger stations used with either the mini-pellet of thesprouted bag encapsulation systems of the prior art. Accordingly, thecapital costs required to achieve a particular productivity level areless than either of these approaches.

Another advantage is that the molding system provides an efficient useof the molding compound. The runners are short from the receptacle tothe cavities. The amount of mold compound left in the package can beminimized by careful design of the plunger so that all or almost all ofthe compound is transferred from the plastic package to the runners. Theamount of sprue or flash left in the runners is far less than a singlepot transfer mold and somewhat less than the mold compound wasteresulting from a multiple pot multiplunger system.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method for packaging mold compound for use inthe encapsulation of integrated circuit lead frame and die assemblies,comprising the steps of:providing a first piece of packaging materialhaving a generally rectangular shape with edges longer than the ends;providing a predetermined quantity of mold compound in a rectangularshape; placing said first piece of packaging material in a loading blockhaving a cavity of predetermined rectangular shape such that said firstpiece packaging material lines the bottom and sides of the loading blockand extends over the sides of the loading block; placing saidpredetermined quantity of mold compound within said loading block andwithin said first piece of packaging material such that the moldcompound is surrounded on the sides and the bottom by said first pieceof packaging material; placing a second piece of packaging materialhaving a generally rectangular shape with edges longer than the endsover said loading block and said first piece of packaging material andsaid predetermined quantity of molding compound; sealing the edges ofsaid second piece of packaging material to the edges of said first pieceof packaging material, said seal being a reversible seal which releasesunder heat; and sealing the ends of said first piece of packagingmaterial so that the mold compound is covered at the ends by said firstpiece of packaging material; wherein said first and second pieces ofpackaging material surround the predetermined quantity of moldingcompound and provide a barrier between said molding compound and air andmoisture.
 2. The method of claim 1, wherein said step of sealing theends of said first piece of packaging material is performed in a vacuumchamber.
 3. The method of claim 2, wherein said step of sealing the endsof said first piece of packaging material comprises sealing the endswith an adhesive.
 4. The method of claim 1, wherein at least one of saidsteps of providing a first piece of packaging material and providing asecond piece of packaging material comprises providing a plastic filmmaterial.
 5. The method of claim 1, wherein at least one of said stepsof providing a first piece of packaging material and providing a secondpiece of packaging material comprises providing a plastic film materialhaving a melting point of at least 200 degrees Celsius.
 6. The method ofclaim 1, wherein at least one of said steps of providing a first pieceof packaging material and providing a second piece of packaging materialcomprises providing a plastic film material of a polyester film.
 7. Themethod of claim 1, wherein at least one of said steps of providing afirst piece of packaging material and providing a second piece ofpackaging material comprises providing plastic film material which willseal to itself when heated.
 8. The method of claim 5, wherein at leastone of said steps of sealing the edges of said second piece of packagingmaterial to said first piece of packaging material comprises applyingheat to said first and second pieces of packaging material until a sealforms between said first and second pieces of packaging material.
 9. Themethod of claim 8, wherein said seal selectively becomes penetrable bythe application of heat.
 10. The method of claim 1, wherein said secondpiece of packaging is opaque.
 11. The method of claim 1, wherein saidsecond piece of packaging is opaque and includes a machine readable codethat indicates said prepackaged mold compound is comprised of aparticular kind of mold compound.
 12. The method of claim 1, whereinsaid step of sealing the ends of said first piece of packaging materialis performed in a pressure chamber providing a pressure of less than 1atmosphere and greater than 15 millibars.
 13. The method of claim 12,wherein said step of sealing the ends of said first piece of packagingmaterial is performed in a pressure chamber providing a pressure ofabout 20 millibars.
 14. The method of claim 1, wherein said step ofproviding a predetermined quantity of mold compound comprises providinga thermoset material.
 15. The method of claim 1, wherein said step ofproviding a predetermined quantity of mold compound comprises providinga resin material.
 16. The method of claim 1, wherein said step ofproviding a predetermined quantity of mold compound comprises providinga resin filler material.